The invention relates generally to a positive collector electrode for batteries with alkaline electrolytes and a process for producing it.
DE-OS No. 35 02 108 has disclosed a positive collector electrode, whose active material comprises a double hydroxide of the general formula EQU [M.sup.2+.sub.( 1-x)Fe.sup.3+ x(OH).sub.2 ].sup.x+ [(x/n)X.sup.n-, yH.sub.2 O].sup.x-
in which the transient parameter x can assume values between 0.05 and 0.4, M.sup.2+ is an oxidizable and reducible cation, and X.sup.n- is at least any desirable anion species serving for charge compensation of the complex double hydroxide-type cation. M.sup.2+ is perferably Ni.sup.2+ or Co.sup.2+, while X.sup.n- may be the anions NO.sub.3-, SO.sub.4.sup.2- or CO.sub.3.sup.2-.
The effort to stem the harmful effect of iron contamination on the function of the nickel electrode lead to the discovery of this double hydroxide and the idea that it should be used as the active material for positive electrodes in alkaline Ni-Fe batteries. In the case of the Edison battery, the principal source of iron contamination is, of course, its negative iron/iron hydroxide electrode. Additionally the various electrode reinforcements in Ni/Cd batteries, whether they are supports made of nickel-plated steel strip or nickel-plated steel fiber mats, will also give rise to the release of iron, as soon as the nickel plating has became defective or porous.
The harmful effect of iron is manifested by a reduction of the charging efficiency of the nickel electrode in a state of electrode oxidation that does not correspond to full charging. This effect is assumed to be caused by the electrophoretic deposition of colloidal iron aquoxide particles containing trivalent iron or iron of lower valency on the nickel hydroxide surface during the charging process, since these particles impart a lower oxygen overvoltage to the electrode. As a result, the electrode takes up less charge. DE-OS No. 35 20 108 discusses some of the reference sources which prove this hypothesis and also explains the formation of the iron aquoxide particles.
The close crystal chemical similarity to the mineral pyroaurite is very significant for this known double hydroxide; they have the same double layer structure and analogous composition, and the position of M.sup.2+ in the complex cation is occupied by Mg.sup.2+ and 1/nX.sup.n-1 by 1/2CO.sub.3.sup.2-. This double layer structure (cf. Allmann, R.: Chimia 24, 99-108 (1970)) derives from the fracture lattice of Beta-Ni(OH).sub.2 and thereby the nickel layers in the M(OH).sub.2 layers (M=Ni) can also be occupied in a statistical distribution by other cations M.sup.2+ and M.sup.3+ so long as they are approximately of equal size. The charge excess introduced by the highly charged trivalent metal cations into the principal layers is now equalized by the X.sup.- anions. This charge equalization is facilitated by the hydroxyl ions of a principal layer which carry a charge -1 due to a reduction of their bond strength corresponding to a charge &lt;1, because of the altered environment of M.sup.2+ and M.sup.3+ ions. This enables these hydroxyl ions to compensate charges &gt;1, and for the H atom of an M--OH bond, to be partially bound by another strong negative atom X. The result of this is the formation of hydrogen bridges O--H . . . X. Together with H.sub.2 O molecules, X.sup.- ions are pushed between the original brucite layers, so that an ionic structure with a succession of principal layers consisting of [M.sup.2+.sub.1-x M.sup.3+ x(OH).sub.2 ].sup.x+ -layer cations and [x/nX.sup.n-, yH.sub.2 O].sup.x- intermediate layer anions (hence "double layer structure") is formed.
It was demonstrated by experiments carried out with electrochemical test cells of the Ni/Fe or Ni/Cd system that electrodes were immune to iron contamination when produced from such a double hydroxide, that is with the empirical composition of Ni.sub.4 Fe(OH).sub.10 NO.sub.3 (which is obtained according to the general formula presented earlier) if x=0.2, when used in place of ordinary nickel hydroxide electrodes. This insensitivity was shown by the fact that the current efficiencies in the cycling experiment were higher from the beginning in the case of double hydroxide electrodes and decreased much less with increasing cycling duration than in the case of the corresponding reference electrodes comprising of 100% iron-free Ni(OH).sub.2.
A similar behavior was also shown by double hydroxide electrodes which contained SO.sub.4 or CO.sub.3, instead of NO.sub.3, which had been prepared by simultaneous precipitation of Ni.sup.2+ and Fe.sup.3+ ions from solutions of the corresponding metal salts by potassium hydroxide, and in which nickel sulfate or nickel carbonate was used as the starting material, in addition to the corresponding iron compounds.
It was also found that the percentage of Ni calculated as pure Ni(OH).sub.2 in the double hydroxide ensures, at least at the beginning of the cycling test, a current efficiency that is very close to the theoretical Faraday efficiency of 289 mAh/g Ni(OH).sub.2. This suggest that the nickel is charged to a state beyond the trivalent state in the presence of iron, which does not itself participate in the redox processes.
The behavior of the positive double hydroxide electrode, which is evidently unaffected by the harmful effects of iron, was taken into account in naming the "siderophile electrode" (siderophile =iron-friendly). Nevertheless, better capacity utilization during longer cycle lives, which is of great practical interest, has continued to be desirable.